1. Field of the Invention
This invention relates in general to valve stem seals and, in particular, to a valve vented redundant stem seal system.
2. Brief Description of Related Art
A gate valve has a body with a central cavity that is intersected by a flow passage. A gate moves within the cavity between the open and closed positions. The gate has a hole through it that aligns with the flow passage while in the open position. The gate may be of a split type, comprising two halves or it may comprise a single slab. A stem extends into engagement with the gate for moving the gate between open and closed positions. The cavity has a central portion, which is intersected by the flow passages, and a stem portion that extends from the central portion.
In one type, the valve stem extends into rotatable engagement with a threaded nut or sleeve secured to the gate. Rotating the valve stem causes the gate to move linearly. In another type, the valve stem does not rotate. A threaded nut sleeve mounted in the bonnet of the valve engages the valve stem, and when rotated, causes the valve stem to move linearly. The threads of the sleeves and stem may slide against each other, or they may employ balls between the grooves for reducing friction.
Gate valves may be operated manually, such as with a wheel mounted to the valve stem or the nut sleeve. It is also known to utilize a remote operated vehicle (ROV) to engage and rotate a stem or nut sleeve. Hydraulically powered actuators are also utilized wherein a piston moves the valve stem linearly without rotation. Electrical actuators are also known that employ an electrical motor and a gear train to rotate a stem or nut sleeve to cause movement of the gate.
Typically, a seal in the valve stem portion of the cavity engages the valve stem to seal pressure within the cavity. The pressure exerted on the valve stem seal can be quite great, leading the valve stem seal to fail. When the valve stem seal fails it will allow fluid or gas to flow out of the valve around the valve stem. Some prior art embodiments provide a second seal that will seal the valve stem to the valve body in the event the primary seal fails. This provides redundancy within the valve stem seals that increases the reliability of the seals.
Where a second seal is used, in the event the first seal leaks, a volume of fluid will be held between the failed first seal and the second seal. Depending on the size of the spacing between the first seal and the second seal, the volume can be quite small. The small volume size allows fluid pressure within the volume to rapidly reach the internal system pressure. This is not a rare condition as many of the environments in which the valves are placed are subject to extreme pressure loads for many years, greatly increasing the likelihood that the primary seal will leak or fail. When the valve is opened, pressure may then quickly vent out past the leaking seal. This can cause further damage to the primary seal so that what had been a small leak becomes a larger failure of the primary seal. A larger failure of the primary seal may eliminate any benefit the leaking primary seal provided, increasing the likelihood of subsequent secondary seal failure. Thus, there is a need for a stem seal with high reliability brought by redundant seals, while allowing for the seals to vent in instances of leakage past the primary seal.